This invention concerns the utilization of the heating values of carbonaceous fuels for the production of useful thermal, mechanical or electrical energy.
Burning coal to generate steam is one of the oldest of the industrial arts. Numerous inventions have been applied to improving its efficiency and alleviating the co-production of noxious smoke which tends to contain unburned fuel, finely powdered ash and oxides of sulfur and nitrogen. Nevertheless, even with the latest technology, coal is considered a dirty fuel capable only with great difficulty and expense of complying with increasingly stringent air pollution standards.
The high cost of removing sulfur oxides from conventional flue gasses has resulted in a spread between the prices of low and high sulfur coals. Moreover, the former are found, for the most part, in western states remote from the areas of greatest energy need. Thus, the market price structure provides economic incentive for a process able to produce steam and power from high sulfur coals without polluting the air.
Combustion of coal in conventional ways creates temperatures well above 2000 degrees F. Apparatus must therefor be constructed of expensive materials capable of withstanding such temperatures. Moreover, components of the ash frequently melt (slag) forming deposits which foul parts of the apparatus. A further undesirable consequence of the usual combustion temperatures is the inadvertent formation of nitrogen oxides, pollutants which cannot be removed economically with available technology.
Generation of high pressure steam, as utilized in modern power plants, does not inherently require such high temperatures since the boiling point of water at 2000 pounds per square inch is only about 635 degrees F. and at 3000 pounds per square inch less than 700 degrees F.
Some experimental combustion systems, particularly those employing fluidized beds of finely divided solids at elevated pressure, permit combustion in a lower temperature range, typically 1500 to 1700 degrees F. Although nitrogen oxides are thus largely avoided, expensive temperature-resistant construction is still required and new difficulties associated with the maintenance of fluidized solids properties, erosion and removal of dust from gas streams are entailed.
It has also been proposed to burn coal without air pollution by the indirect means of first converting it to liquid or gaseous fuel which can be desulfurized by known technology. These techniques also employ high temperatures and generally share serious economic and operational drawbacks associated with coal's tendency to cake and stick when heated, the formation of tarry residues and difficulties with erosion and dust control. They are further burdened by low overall thermal efficiencies.
It has long been known that liquid water accelerates the reaction between coal and atmospheric oxygen so that it is at least possible that finely divided coal in the form of an aqueous slurry can be burned at temperatures much lower than those of conventional combustion, perhaps 550 to 700 degrees F. The laws of thermodynamics teach us that such a process would oxidize sulfur compounds in the coal to the trioxide rather than the dioxide encountered in conventional flue gasses and that the formation of oxides of nitrogen would be essentially nil. Sulfur trioxide is highly soluble in water, combining with it to form sulfuric acid, whereas the solubility of the dioxide is comparatively low. Thus, the flue gas from such a process could be essentially free of oxides of both sulfur and nitrogen. Moreover, particles of ash would be expected to remain with the aqueous phase.
In the indicated temperature range water has a vapor pressure of approximately 1000 to 3000 pounds per square inch. Operating pressure necessary to maintain liquid phase would be even higher because of the partial pressure effect of air or flue gas. The costs of compressing combustion air to such pressure levels would be a serious handicap for such a process. Also, water's critical temperature of about 705 degrees F., above which liquid phase does not exist, limits the level at which useful heat can be delivered.
It is interesting and significant that sulfur contained in a fuel charged as a slurry in a liquid containing at least some water, and burned at relatively low temperature, becomes sulfuric acid. Moreover, if not neutralized or withdrawn, this acid, being substantially less volatile than water, will accumulate and become increasingly concentrated. It happens that this familiar reagent and chemical commodity has properties useful to creating a new and novel slurry-phase combustion process, capable of producing pollution-free flue gas without the necessity of high operating pressure and its attendant air compression costs.
Among the pertinent properties of sulfuric acid is extreme stability to heat and oxidation. Secondly, its vapor pressure is quite low so that temperatures suitable for the generation of high pressure steam can be achieved at relatively low levels of superimposed pressure. Thirdly, it is a powerful oxidizing agent in its own right and may be expected to accelerate the combustion reactions. An important economic consideration is that, as a by-product, its supply does not represent an operating cost. On the contrary, a modest surplus, the amount being a function of the sulfur content of the fuel, would be available for sale.
Also pertinent are the chemical interactions among sulfur dioxide, oxygen, sulfur trioxide, water vapor and sulfuric acid. The thermodynamic equilibria among these components have been accurately determined and form part of the fundamental data upon which the commercial manufacture of sulfuric acid is based. In general, increased pressure, oxygen and water concentrations and decreased temperature favor the trioxide or sulfuric acid form, all of which conditions can be manipulated to limit the production of the undesirable dioxide to a very low tolerance.
In the large body of art concerning the control of pollution from conventional coal combustion, sulfuric acid appears in at least two roles. In one, flue gas containing sulfur dioxide is passed, at a temperature substantially lower than the combustion temperature, over a catalyst similar to that used in the contact process of sulfuric acid manufacture. The sulfur dioxide is thereby converted to the trioxide which unites with water vapor to become liquid sulfuric acid which is then separated from the flue gas. In another role, flue gasses are scrubbed with strong sulfuric acid which acts as a solvent for sulfur dioxide and oxides of nitrogen. Both of these processes rely on the extreme stability and low vapor pressure of this acid.
There is also a sizeable body of art concerning the concentrating of dilute solutions of sulfuric acid which furnishes information useful to controlling the water content of an acid solution used as a slurrying medium in a novel combustion process. In some examples of this art water is vaporized from the acid by direct contact with hot flue gas and, in at least one such process, this contact results from combustion (in the conventional temperature range) under the liquid surface, a technique referred to as "submerged combustion". Experience with the operation of these acid concentrating processes is also useful to the selection of materials of construction for an acid slurry combustion process.